Final answer:
The frequency of solar and lunar eclipses, and the appearance of lunar phases were observations that led ancient Greeks to conclude the Moon was nearer to Earth than the Sun.
Step-by-step explanation:
Two observations that the ancient Greeks could have made to logically deduce that the Moon must be nearer to Earth than the Sun are the frequency of solar and lunar eclipses and the appearance of lunar phases. First, during a solar eclipse, the Moon passes between the Earth and the Sun, which would not be possible if the Sun were closer. This would lead to the logical assumption that the Moon is nearer to our planet. Second, the phases of the Moon, which change systematically over a month, signify that the Moon is close enough to be illuminated differently by the Sun at various angles. Both of these regular astronomical events would have been a clear indication to the ancient Greeks of the relative proximity of the Moon to the Earth compared to the Sun.